Persistent memory for key-value storage

ABSTRACT

Using persistent memory for key-value storage is disclosed, including: receiving a put key operation including a key and a corresponding value; directly storing the key at a persistent memory media using a physical address that is determined based at least in part on the key, wherein the persistent memory media supports memory persistence and in-place updating; directly storing the value to the persistent memory media; receiving a get key operation including the key; and directly accessing the persistent memory media using the physical address that is determined based at least in part on the key to retrieve the value corresponding to the key.

BACKGROUND OF THE INVENTION

Existing key-value databases running on traditional memory/storagehierarchies suffer from significant inefficiencies. For example, atraditional memory/story hierarchy includes a combination of DynamicRandom Access Memory (DRAM) and Solid State Drive (SSD) and/or Hard DiskDrive (HDD). Key-value data from an application goes through a long pathbefore it arrives at the storage media, and each access incurs multipledisk I/Os.

In some example processes of storing key-value data in traditionalmemory/storage hierarchies, key-value records are first stored in amemory data structure before they are written to disk files through a“compaction” process. To improve response time, disk files are organizedinto multiple levels (e.g., level-0 through level-3) with differentsizes, with most frequent (and recent) used records stored in level-0initially. When the files of a level fill up, their data is moved to thenext level also through the compaction process. As such, a look up of akey could involve reading multiple files across several levels.Furthermore, some data is duplicated between multiple levels andtherefore wastes some of the storage space. Due to these issues, thetraditional key-value storing technique suffers from high overhead, highread/write amplification, and space amplification (wasted space).

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a diagram showing various example pairs of key-values storedin a key-value database.

FIG. 2 is a diagram showing a conventional key-value storage scheme.

FIG. 3 is a diagram of an embodiment of a system for a persistent memoryfor key-value storage.

FIG. 4A is a flow diagram showing an embodiment of a process for storinga received key-value pair at a persistent memory for key-value storage.

FIG. 4B is a flow diagram showing an embodiment of a process forretrieving a stored value corresponding to a received key at apersistent memory for key-value storage.

FIG. 4C is a flow diagram showing an embodiment of a process for usingpersistent memory for key-value storage.

FIG. 5 is a flow diagram showing an embodiment of a process formaintaining a page usage data structure.

FIG. 6A shows example metadata that is stored for a key page.

FIG. 6B shows example metadata that is stored for a value page.

FIG. 7 is a flow diagram showing an embodiment of a process for storinga new key-value pair in a KVPM.

FIG. 8 is a flow diagram showing an embodiment of a process forperforming an in-place update for a stored key.

FIG. 9 is a flow diagram showing an embodiment of a process forperforming an out-of-place update for a stored key.

FIG. 10 is a flow diagram showing an example of a process for performinga put key operation.

FIG. 11 shows an example of the result of using the put key operation tostore a new key-value pair in pages of the KVPM.

FIG. 12 shows an example of the result of using the put key operation tostore a new value for a previously stored key in pages of the KVPM inthe in-place update mode.

FIG. 13 shows an example of the result of using the put key operation tostore a new value for a previously stored key in pages of the KVPM inthe out-of-place update mode.

FIG. 14 is a flow diagram showing an embodiment of releasing a snapshotversion of a value corresponding to a stored key.

FIG. 15 shows an example of pointers between a key page and value pagesassociated with storing various versions of the value corresponding to akey.

FIG. 16 shows an example of releasing a snapshot version of a valuecorresponding to a key.

FIG. 17 is a flow diagram showing an embodiment of a process forretrieving from a KVPM a stored value corresponding to a key.

FIG. 18 is a flow diagram showing an example of a process for performinga get key operation.

FIG. 19 shows an example of the result of using the get key operation toretrieve from the pages of a KVPM a stored value corresponding to agiven key.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

FIG. 1 is a diagram showing various example pairs of key-values storedin a key-value database. Key-values may be stored by a key-valuedatabase. Examples of key-value database software include NoSQLdatabases such as Redis® and RocksDB®. Each value object is storedcorresponding to a key and the value object may be retrieved from thedatabase using the key that uniquely identifies that object. Referringto the key-value pairs shown in FIG. 1, the key “Key1” may be used toobtain the value “111, 222, 333.” Similarly, the key “Key2” may be usedto obtain the value “ABC, DEF, 123,” and so forth.

Key-value storage is widely used in cloud computing, web applications,and enterprise applications. For example, key-value storage may be usedby backend storage engines for other types of storage. Key-value storagemay be used to store the metadata of items, for example. In a specificexample, a key could be the International Standard Book Number (ISBN) ofa book and the corresponding value may include the author of the book,the title of the book, and the publisher of the book. Numerous otheruses exist.

FIG. 2 is a diagram showing a conventional key-value storage scheme. Aswill be described below, in the conventional key-value storage scheme,the key-value data that is sent from application 202 goes through a longpath before it arrives at the storage media (NAND flash 212) and eachaccess for a value corresponding to a key incurs multiple disk I/Os.

Take RocksDB® as an example: a key-value pair sent from application 202to be stored at NAND flash 212 is first stored in in-memory datastructure 204 and is written to various levels of files within files infile system 206 in a compaction process. In the example of FIG. 2, thekey-value data is first written to Static Sorted Table (SST) Files atLevel-0. When the Level-0 SST files fill up, the key-value data iscompacted into (larger) Level-1 SST files, and so on and so forth. Dueto key-value data being stored across different levels, a look up of avalue corresponding to a key may involve multiple reads from filesacross several levels.

Once the key-value data fills up the last level of SST files, they areconverted into regular disk I/Os by using block device driver 208 totranslate a Logical Block Address (LBA) of a set of key-value data to acorresponding Physical Block Address (PBA) in NAND flash 212. As such,Flash Translation Layer (FTL) and garbage collection 210 is configuredto maintain a large logical-to-physical mapping table internally. Forexample, Flash Translation Layer (FTL) and garbage collection 210 may beconfigured as separate FTL and garbage collection modules. The I/Ocommands will go through several firmware modules such as FTL andgarbage collection 210 before being written to NAND flash 212.

Due to these inefficiencies along the I/O path as described with FIG. 2,conventional SSD-based key-value storage suffers from high overhead andsignificant read/write amplification. And since some data is duplicatedbetween multiple levels of SST files, some of the storage space iswasted.

Furthermore, additional properties of NAND flash storage (e.g., NANDflash 212) cause reads and writes to the NAND flash storage to beinefficient. The first of such inefficiencies is due to the NAND flashstorage's inability to perform overwriting of a previously written page(i.e., an in-place update). Because NAND flash memory does not supportin-place updates, data can only be written to erased pages. Due to theselimitations, data on flash-based SSD is indirect. Indirect data meansthat an LBA received from a host computer must first be mapped to acorresponding PBA on the drive. The storage controller uses an FTL tomanage this LBA→PBA mapping. As such, the FTL needs to maintain alogical-to-physical mapping table in FTL in order to serve read commandsfrom the host computer. As the capacity of a NAND flash storageincreases, the size of the logical-to-physical mapping table alsoincreases. For example, 1 TB of user capacity typically requires 1 GB ofDRAM to hold the logical-to-physical mapping table. This is challengingto the NAND flash storage as the usage of the logical-to-physicalmapping table increases both power consumption and cost.

Another inefficiency of NAND flash storage is the requirement that dataonly be written to erased pages. In NAND flash storage, data can only bewritten to “clean” (i.e., erased) pages. Data in NAND flash memory isread or written in units of pages (typically 4 KB large). Pages areerased in units of Erase Block (EB), which is typically 128 KB or 256KB. If an EB contains useful data, it must be relocated before the EB iserased in order to avoid any data loss. When serving a write commandfrom the host computer, the storage controller always writes data to“clean” (erased) pages and marks overwritten pages as “invalid” in FTL.This process continuously consumes clean pages. Therefore, the storagecontroller must reclaim space occupied by invalid pages in thebackground so that the drive will not run out of clean pages. Thisbackground process is called “garbage collection” (GC). During a typicalGC operation, the storage controller picks a few EBs, moves all validpages in them to clean pages, and then erases these original EBs toproduce clean pages. The GC process is also responsible for distributingP/E cycles across drive capacity in order to achieve the expected lifetime of the NAND flash storage and to avoid wearing out some pages tooearly.

Persistent memory's combination of memory and storage characteristicshas opened new opportunities in software/hardware optimization, enablingnew usage scenarios such as in-memory database, in-memory computation,and fast failure recovery. Persistent memory has properties that avoidsome of the inefficiencies that are associated with the conventionalSSD-based key-value storage. Embodiments of using persistent memory forkey-value storage are described herein. In various embodiments,key-value storage is implemented at a persistent memory. In variousembodiments, “persistent memory” refers to a type of storage that haslow latency, high endurance, supports in-place updates, serves randomaccesses (with similar performance as sequential accesses), and isnon-volatile (e.g., data stored on the persistent memory will not belost in the event that the computing device with which it is associatedis shut down). As such, persistent memory provides benefits of bothmemory and permanent storage. Examples of persistent memory mediainclude Phase Change Memory, ReRAM, and STT-RAM. As will be described infurther detail below, in various embodiments, using persistent memoryfor key-value storage allows for in-place updates of a key-value pairand also directs access to the persistent memory.

FIG. 3 is a diagram of an embodiment of a system with a persistentmemory for key-value storage. In the example, application 302 (e.g.,RocksDB®) is configured to provide a key-value store operation tokey-value persistent memory (KVPM) 304. In various embodiments, akey-value store operation provided by application 302 comprises a “putkey” operation that is associated with storing a given key-value pair, a“get key” operation that is associated with requesting a correspondingvalue for a given key, or any other appropriate operation for accessingor modifying a key-value pair. In some embodiments, a put key operationmay be used to store a new key-value pair or update a new value for apreviously stored key.

The key-value store operation sent by application 302 is received atKVPM interface 306 of KVPM 304. In various embodiments, KVPM interface306 is configured to determine a physical address associated with a pagewithin persistent memory media 310 based on a given key of the received(e.g., put key or get key) operation from application 302. In someembodiments, KVPM interface 306 maps the given key of the operation to aphysical address associated with a page within persistent memory media310. For example, persistent memory media 310 comprises Phase ChangeMemory, ReRAM, and/or SST-RAM. For example, KVPM interface 306 may beimplemented using hardware that is running software and/or firmware, asan application-specific integrated circuit (ASIC), as software, oranother appropriate device executing KVPM code. The physical addressassociated with a page within persistent memory media 310 to which KVPMinterface 306 had mapped the given key is a page at which the keyassociated with the operation is to be stored and/or should be stored,depending on whether the received operation is put key or get key,respectively. Media manager 308 is configured to use the physicaladdress to complete the operation. In some embodiments, media manager308 is implemented using a processor, firmware, and/or controller. Insome embodiments, media manager 308 is configured to perform errorcorrection on values retrieved from persistent memory media 310.

As will be described in further detail below, media manager 308 of KVPM304 handles the keys and values received from application 302 directly.In various embodiments, handling the keys and values received fromapplication 302 “directly” refers to writing to and reading (accessing)from persistent memory media 310 without performing alogical-to-physical address mapping. Instead, KVPM 304 uses the givenkey of an operation to determine a page in persistent memory media 310at/from which to complete the operation. In addition, accesses in KVPM304 exhibit much shorter I/O paths and lower overhead than they would ina conventional key-value store that is implemented by a NAND flashstorage. No compaction among memory table and multiple levels of SSTfiles is needed because persistent memory media 310 is non-volatile.There is no complex FTL or garbage collection either because nological-to-physical mapping is used and also, in-place updates arepermitted by persistent memory media 310.

While not shown in system 300, application 302 may optionally apply acache in volatile memory (DRAM) to hold frequently accessed data (e.g.,frequently accessed key-value pairs). Application 302 may also keepindexing data and Bloom Filters in DRAM to further accelerate accesses.

FIG. 4A is a flow diagram showing an embodiment of a process for storinga key-value pair at a persistent memory for key-value storage. In someembodiments, process 400 is implemented at a KVPM such as KVPM 304 ofFIG. 3.

At 402, a put key operation including a key and a corresponding value isreceived. In various embodiments, a put key operation comprises arequest to store a key-value pair that is included in the put keyoperation. For example, the put key operation is received from anapplication.

For example, the put key is provided by a database application formanaging key-value pairs that include a book ISBN (key) and a book title(value). For instance, the key-value pair provided by the application ina put key operation is “0743273567—The Great Gatsby.”

At 404, the key is directly stored at a persistent memory media using aphysical address that is determined based at least in part on the key,wherein the persistent memory media supports memory persistence andin-place updating. In various embodiments, a key from an operation suchas a put key operation is directly mapped to a page of the persistentmemory media of the KVPM using a hash function. For example, the hashfunction maps at least a portion of the key to a physical address at thepersistent memory media at which to potentially store the key of the putkey operation.

The following is an example formula for mapping a key to a physicaladdress in the persistent memory media:

Key_(address)=(Hash(Key)% NumOfPages)*PageSize  (1)

where K_(eyaddress) is the physical address of a memory page at thepersistent memory media at which to potentially store the key includedin the operation, Hash( ) is a hash function that maps the key to afixed-length value, NumOfPages is the total number of pages in usablecapacity of the persistent memory media, and PageSize is the size ofeach memory page in bytes. K_(eyaddress) is aligned to a page boundaryin the KVPM. An example of Hash( ) is the Jenkins hash function.

Formula (1) is only one example equation associated with a hashingtechnique that can be used to map a key of an operation to a physicaladdress of a page in the persistent memory media and other mappingtechniques may be used in practice.

The key of the put key operation is stored in the page in the persistentmemory media associated with the Key_(address) unless it is determinedthat the page is already storing another, different key. In this event,a hash collision has occurred, in which two or more keys have beenmapped to the same address in the persistent memory media. In someembodiments, when a hash collision occurs, an “open addressing” strategyis used. When the hashed-to page of a key is not available (because italready stores another key, for example), in some embodiments, a probesequence is used to look for an available page in the persistent memorymedia. In various embodiments, a “probe sequence” refers to a selectiontechnique of a next page in the persistent memory media to check foravailability to store a key. For example, a probe sequence may indicateto check the page in the persistent memory media that is a certainnumber of pages (e.g., five pages) away from the current page inquestion. To reduce repeated collisions and clustering, in someembodiments, double hashing is used to derive the probe sequence—theinterval that the probe sequence steps forward each time is calculatedfrom another independent hash function. The probe sequence is followedto identify an available page in the persistent memory media until sucha page is found or the maximum number of attempts to find such a page isreached.

Returning to the previous example, once an available page in the KVPM isdetermined to store the key of the put key operation, the ISBN (key) of“0743273567” will be stored in that page.

Where it is determined that a page in the persistent memory mediaalready stores the given key of the put key operation, it is determinedthat the put key operation is associated with updating a previouslystored key-value pair with a new value. In various embodiments, eitherin-place (e.g., overwriting of the previous value) updating may beperformed or out-of-place (e.g., not overwriting of the previous value)updating may be performed at the persistent memory media, as will bedescribed in further detail below.

At 406, the value is directly stored at the persistent memory media.After the key is stored in a page in the persistent memory media, thevalue corresponding to the key is stored in one or more other pages inthe persistent memory media. If the value is too large to be stored inone page in the persistent memory media, the value is stored acrossmultiple pages (not necessarily contiguous pages) in the persistentmemory media. In some embodiments, the pages in which to store the valuemay be selected using any appropriate technique so long as they arefree/unused for storing other data. For example, pages nearby the keypage may be linearly searched through until a free page is found forstoring the value. In another example, a random page in the KVPM isselected until a free page is found for storing the value.

In various embodiments, the page at which the key is stored (which issometimes referred to as the “key page”) is linked to the first pagethat stores the value (which is sometimes referred to as a first “valuepage”) corresponding to the key. In some embodiments, the key page islinked to the first value page by storing a pointer to the first valuepage in the metadata portion of the key page in the persistent memorymedia. In some embodiments, a pointer to a page comprises a physicaladdress of the page. In various embodiments, the metadata portion of apage in the persistent memory media is the out-of-band (OOB) space ofthe page. In various embodiments, the OOB space of a page in thepersistent memory media is the portion of the page that is in additionto the portion used to store user data and is only visible to the KVPMinternally. The page(s) that store the value are also linked togetherthrough storing pointers from one value page to another in the metadataportion (e.g., the OOB space) of the value pages in the persistentmemory media. The series of linked value pages associated with storing avalue is sometimes referred to as a “value page chain.”

Returning to the previous example, title (value) of “The Great Gatsby”corresponding to the ISBN (key) “0743273567” will be stored across oneor more allocated value pages. The key page that stores “0743273567”will include a pointer to at least the first value page of the valuepage chain that stores “The Great Gatsby.” Each value page that stores aportion of “The Great Gatsby” will also include a pointer that points tothe next value page of the chain, if any.

FIG. 4B is a flow diagram showing an embodiment of a process forretrieving a stored value corresponding to a received key at apersistent memory for key-value storage. In some embodiments, process410 is implemented at a KVPM such as KVPM 304 of FIG. 3.

At 412, a get key operation including a key is received. Sometime afterthe key-value pair of the put key operation is stored at the persistentmemory media (e.g., using a process such as process 400 of FIG. 4A), aget key operation associated with retrieving a stored valuecorresponding to a stored key-value pair is received. The get keyoperation includes the key for which the corresponding value isrequested.

For example, the get key operation is received from the application formanaging key-value pairs that include a book ISBN (key) and a book title(value). For instance, to retrieve the book title (value) correspondingto the ISBN (key) of “0743273567,” that key is provided by theapplication in a get key operation.

At 414, a persistent memory media is directly accessed using a physicaladdress that is determined based at least in part on the key to return astored value corresponding to the key, wherein the persistent memorymedia supports memory persistence and in-place updating. In variousembodiments, the given key of the get key operation is mapped to thesame physical address in the persistent memory media at which the key isexpected to be stored. In various embodiments, the same hash functionthat was used to store (e.g., complete the put key operation) theoperation for a key is used to perform the get key operation.

For example, if formula (1) above was used to store the key in thepersistent memory media in a put key operation, then formula (1) aboveis also used to read the stored value corresponding to the given key ofthe get key operation. Similar to the description above, in the eventthat the page at the physical address to which the given key was hasheddoes not store the given key, then the same probe sequence that was usedto resolve the hash collision for the put key operation is used tosearch for the correct page at which the given key was stored for theget key operation. Once the key page is identified, the pointer to thefirst value page that is stored in the key page is used to locate thefirst value page in the persistent memory media. The first value pagealso includes a pointer to the next value page, if one exists, and soforth. As such, the value corresponding to the given key of the get keyoperation may be retrieved from the chain of one or more value pages forthe application.

Returning to the previous example, once the key page in the KVPM that isused to store ISBN (key) of “0743273567” is determined, the pointerstored in the key page to the first value page that stores thecorresponding value is used to locate the first value page. The portionof the value (“The Great Gatsby”) is read from the first value page andthe pointer stored in each value page that points to the next value pagein the chain is used to locate each subsequent value page, if oneexists, until the complete value of “The Great Gatsby” is retrieved.

FIG. 4C is a flow diagram showing an embodiment of a process for usingpersistent memory for key-value storage. In some embodiments, process420 is implemented at a KVPM such as KVPM 304 of FIG. 3.

At 422, a put key operation including a key and a corresponding value isreceived.

At 424, the key is directly stored at a persistent memory media using aphysical address that is determined based at least in part on the key,wherein the persistent memory media supports memory persistence andin-place updating.

At 426, the value is directly stored at the persistent memory media.

At 428, a get key operation including the key is received.

At 430, the persistent memory media is directly accessed using thephysical address that is determined based at least in part on the key toreturn the stored value corresponding to the key.

FIG. 5 is a flow diagram showing an embodiment of a process formaintaining a page usage data structure. In some embodiments, process500 is implemented at KVPM 304 of FIG. 3.

Process 500 describes an example process of maintaining a data structurethat tracks which memory pages of the KVPM are used or unused/freeduring runtime.

At 502, a page usage data structure is generated at a KVPM at boot time.At the boot time of the KVPM, the memory pages of the KVPM are scannedto determine which pages are used/cannot be allocated (e.g., have beenwritten to with data that is to be retained) and which pages arefree/unused/can be allocated (e.g., have not been written to and/or havebeen written to but with data that is not to be retained). For example,data that is to be retained in a page is data that is not requested bythe application to be deleted/released while data that is not to beretained is data that is requested by the application to bedeleted/released. In some embodiments, a page's used or unused status isdetermined based at least in part on scanning a metadata field (e.g.,the page type field in the OOB space) of that page. In some embodiments,the page usage data structure comprises a bitmap that is stored in aDRAM, where each memory page has a corresponding bit in the bitmap thatindicates whether the page is free and therefore available to beallocated for a new write (e.g., a new key or a new value), or not.

At 504, the page usage data structure is updated in response to updatesin page allocation in the KVPM during runtime. During runtime, as newkey-value pairs are written to and released from the pages of the KVPM,the page usage data structure is continuously/dynamically updated totrack which pages are currently unused or used, as will be described infurther detail below.

FIG. 6A shows example metadata that is stored for a key page and FIG. 6Bshows example metadata that is stored for a value page. In variousembodiments, the metadata that is stored for each key page (as mentionedabove, a key page is a page in the KVPM that stores a key) and themetadata that is stored for each value page (as mentioned above, a valuepage is a page in the KVPM that stores at least a portion of a valuecorresponding to a key) are stored in the page's respective OOB space.

As shown in FIG. 6A, the example metadata that is stored in the key pageincludes the following:

Page type field: this field is set to “key page” to indicate that thisis a key page.

Pointer to current value page chain: this field includes a pointer to a(e.g., physical address of) (e.g., first) value page in the KVPM thatstores a value corresponding to the key.

Snapshot count: this field includes a value associated with how manysnapshot versions of values exist for this key-value record. Thiscounter increments every time a snapshot version is created (i.e., anupdate with out-of-place mode is performed), and decrements when asnapshot version is released (e.g., a snapshot version is marked fordeletion). Details of snapshot versions are described in greater detailbelow.

Delete flag: this field indicates whether a key-value record is markedfor deletion from the KVPM. In some embodiments, deletion is deferred ifthere are still unreleased snapshot versions (i.e., the snapshot countis non-zero). Once all snapshot versions of values for a key arereleased, the key-value record is deleted with all pages freed/released.

Statistical information: this field stores statistical information totrack usage of the key-value record. For example, this field may storethe read/write count of the key-value record.

As shown in FIG. 6B, the example metadata that is stored in the valuepage includes the following:

Page type field: this field is set to “value page” to indicate that thisis a value page.

Version number of the value page chain: this field includes a versionnumber of the value that is stored in the current value page. The valuein this field is assigned by a host. For example, the version number mayindicate “current” for the current version of the value or a numericvalue associated with the snapshot version with which the value isassociated.

Pointer to next value page in chain: If the current value page is thelast page of a value page chain, then this field stores a pointer thatis set to an invalid address.

Pointer to previous version of value page chain: If no previous versionof a value exists, then this field stores a null pointer. This fieldonly stores data for a value page that is the first page of a value pagechain.

Back pointer: For the first page in a value page chain, this fieldincludes a pointer that points back to the key page. For pages otherthan the first page in a value page chain, this field includes a pointerthat points back to the previous page in the chain.

For unused (free) pages, their “page type” fields are set to “unused”with all other metadata fields reset to a pre-defined “reset” state.

FIG. 7 is a flow diagram showing an embodiment of a process for storinga new key-value pair in a KVPM. In some embodiments, process 700 isimplemented at KVPM 304 of FIG. 3. In some embodiments, process 400 ofFIG. 4A or steps 422-426 of process 420 of FIG. 4C are implemented atleast in part using process 700.

At 702, a put key operation including a key and a corresponding value isreceived. For example, the put key operation is received from anapplication. In process 700, the received put key operation isconfigured to store a new given key-value pair at the KVPM.

At 704, a physical address of a KVPM is determined based at least inpart on the key. In various embodiments, a formula including a hashfunction (e.g., formula (1) above) is used to compute a physical addressof the KVPM based on at least a portion of the key.

At 706, the key is stored in a page associated with the physical addressof the KVPM. The page in the KVPM that is associated with the physicaladdress that is determined by the hash function is first checked forwhether it is unused/free. For example, whether a page is unused or usedmay be determined by checking its “page type” field, as described abovewith FIGS. 6A and 6B. For example, if the page type field indicates“unused,” then the page is free; but if the page type field indicates avalue other than “unused” (e.g., “value page,” “key page”), then thepage is used. If the page is used, then a hash collision may haveoccurred and another unused page may be determined using a predeterminedprobe sequence, in some embodiments. In various embodiments, thepredetermined probe sequence may comprise any technique for selecting anext page (e.g., relative to the current page that is determined) forwhich the page type (e.g., used or unused) is to be checked. Once anunused page is determined relative to the physical address that isdetermined by the hash function, the key is stored in that page. Invarious embodiments, the metadata stored in the 00B space of the keypage is updated such that the page field type is “key page.”

In some embodiments, more than one key may be stored at a single page ofthe KVPM (e.g., based on a user configuration of the KVPM). Therefore,in some embodiments, in the event where the page of the KVPM associatedwith the physical address determined by the hash function alreadyincludes one or more existing keys, the new key will still be stored atthat page. Metadata corresponding to each key of the page will be storedin the OOB space of that page. However, for purposes of illustration, apage is configured to store one key in embodiments described herein,unless noted otherwise.

At 708, the value corresponding to the key is stored in a set of one ormore pages of the KVPM. It is determined whether the value needs to bestored in a single page of the KVPM or more than one page. For example,a page in the KVPM may store up to 4 KB of user data so the total numberof pages that will be needed to store the value will be the size of thevalue divided by 4 KB. In some embodiments, a user may configure arestriction on which set of unused pages of the KVPM (e.g., pagesassociated with a certain range of addresses) may be allocated forstoring the key. The number of unused pages that are needed to store thevalue are allocated according to any restrictions on such value pagesand then used to store the value corresponding to the key. In variousembodiments, the pages that are used to store the value corresponding tothe key are not required to be contiguous. In various embodiments,allocating a value page includes determining unused pages using the pageusage data structure and setting the page type field of a determinedunused page to “value page.”

At 710, associating data between the key page and the corresponding setof one or more value pages is stored. In various embodiments, pointersbetween the key page and the value pages are stored in the metadatastored in the pages' OOB space. Specifically, in some embodiments, thephysical address of the page in the KVPM that is the (first) page of thevalue page chain is stored in the OOB space (e.g., in the “pointer tocurrent value page chain” field of the key page in FIG. 6A) of the keypage. In some embodiments, the physical address of the page in the KVPMthat is the next page, if any, of the value page chain is stored in theOOB space (e.g., in the “pointer to next value page in chain” field ofthe value page in FIG. 6B) of a value page. In some embodiments, thephysical address of the page in the KVPM that is a previous page, ifany, of the value page chain is stored in the OOB space (e.g., in the“back pointer” field of the value page in FIG. 6B) of a value page thatis other than the first page of the value page chain. In someembodiments, the physical address of the page in the KVPM that is thekey page is stored in the OOB space (e.g., in the “back pointer” fieldof the value page in FIG. 6B) of the first value page of the value pagechain. In some embodiments, a version number of the value page chainthat indicates that the value page chain is the current version of thevalue corresponding to the key is stored in the 00B space (e.g., in the“version number of the value page chain” field of the value page in FIG.6B) of each value page of the value page chain.

At 712, a page usage data structure that tracks page allocation isupdated. The page usage data structure is updated to indicate that thepages associated with storing the key and value of the key-value pairare now used.

In various embodiments, when a previous value of a stored key is updatedto a new value, the KVPM supports both in-place and out-of-place updatemodes.

When an in-place update is used, at least a portion of the page(s) thatwere used to store a previous value corresponding to a stored key isoverwritten with the new/updated value of the stored key. In the eventthat fewer pages are needed to store the new/updated value of the storedkey, the additional value page(s) are released (e.g., set to be unusedso that they can be allocated for new writes). For example, an in-placeupdate may be desirable when it is not needed or desired to preserve aprevious version of a value corresponding to a key. FIG. 8 is an exampleflow of an in-place update.

When an out-of-place update is used, new value pages are allocated tostore the current version of the value of a stored key and the valuepages that store a previous value corresponding to the key become aprevious version. The previous value is sometimes referred to as a“snapshot version.” For example, a snapshot of a previous value may beretained in the event that the host requests to restore that previousversion of the value for whatever reason. In a specific example, a hostmay wish to create a snapshot version of a value corresponding to astored key prior to a system update. In the event that the system updatedoes not succeed, the snapshot version of the value may be recovered tohelp restore the system to a stable state prior to the updating. After asnapshot version of a value is generated, the key page is updated topoint to the new value pages that store the current version of thevalue. A pointer back to the value pages storing the older, snapshotversion of the value is stored in the new value page chain storing thecurrent version of the value, so that the host can access previoussnapshots easily. FIG. 9 is an example flow of an out-of-place update.

A host can dynamically select the update mode (e.g., the in-place modeto avoid creating snapshots and the out-of-place mode to intentionallycreate snapshots) at runtime, and at a per-key granularity, for example.While the in-place mode is more space efficient because it reusesprevious value pages and avoids the creation of snapshot versions, pastvalue states are lost when new values are written to the KVPM for a key.While the out-of-place mode is less space efficient because it creates asnapshot of a past value each time a new value is to be written to theKVPM for a key, past value states are retained and available to berestored, should the need arise. In some embodiments, the update modemay be set by setting the value of a global variable associated with theupdate mode. This allows the host to create fine-grained snapshotson-demand. For example, the host can enable out-of-place updates onrelevant key/value records before performing a critical update, andswitch to the in-place update mode when snapshotting is not needed. Thisnot only provides much more flexibility, but also reduces space wastecaused by unused/unnecessary snapshot copies.

FIG. 8 is a flow diagram showing an embodiment of a process forperforming an in-place update for a stored key. In some embodiments,process 800 is implemented at KVPM 304 of FIG. 3. In some embodiments,at least part of process 400 of FIG. 4A or steps 422-426 of process 420of FIG. 4C are implemented using process 800.

At 802, a put key operation including a key and a corresponding newvalue is received. For example, the put key operation is received froman application. In process 800, the received put key operation isconfigured to store a new value, in-place, corresponding to a previouslystored key at the KVPM.

At 804, a physical address of a KVPM is determined based at least inpart on the key. In various embodiments, a formula including a hashfunction (e.g., formula (1) above) is used to compute a physical addressof the KVPM based on at least a portion of the key. The page in the KVPMthat is associated with the physical address that is determined by thehash function is first checked for whether it contains the key includedin the put key operation. In some embodiments, the predetermined probesequence is used to select the next page to check for the key includedin the put key operation, until the appropriate key page is located.

At 806, associating data between a key page and a corresponding set ofone or more value pages associated with storing a previous valuecorresponding to the key is used to identify the corresponding set ofone or more value pages. The metadata (e.g., pointers) stored in the 00Bspace of the located key page is used to locate the value pages thatstore the previous version of the value corresponding to the key.

At 808, a new value corresponding to the key is stored by overwriting atleast some of the corresponding set of one or more value pages. As manyof the identified value pages that are needed to store the new value ofthe key are overwritten by new value. In the event that more value pagesare needed to store the new value corresponding to the key, additionalpages are allocated for doing so. In the event that fewer value pagesare needed to store the new value corresponding to the key, theremaining value pages are released.

At 810, a page usage data structure is updated. The page usage datastructure is updated to indicate that the additional page(s) associatedwith updated values are now used or that the released value pages arenow unused.

FIG. 9 is a flow diagram showing an embodiment of a process forperforming an out-of-place update for a stored key. In some embodiments,process 900 is implemented at KVPM 304 of FIG. 3. In some embodiments,at least part of process 400 of FIG. 4A or steps 422-426 of process 420of FIG. 4C are implemented using process 900.

At 902, a put key operation including a key and a corresponding newvalue is received. For example, the put key operation is received froman application. In process 900, the received put key operation isconfigured to store a new value, out-of-place, corresponding to apreviously stored key at the KVPM.

At 904, a physical address of a KVPM is determined based at least inpart on the key. In various embodiments, a formula including a hashfunction (e.g., formula (1) above) is used to compute a physical addressof the KVPM based on at least a portion of the key. The page in the KVPMthat is associated with the physical address that is determined by thehash function is first checked for whether it contains the key includedin the put key operation. In some embodiments, the predetermined probesequence is used to select the next page to check for the key includedin the put key operation, until the appropriate key page is located.

At 906, the new value corresponding to the key is stored in a new set ofone or more value pages of the KVPM. It is determined whether the newvalue needs to be stored in a single page of the KVPM or more than onepage. In some embodiments, a user may configure a restriction on whichset of unused pages of the KVPM (e.g., pages associated with a certainrange of addresses) may be allocated for storing the key. The number ofunused pages that are needed to store the new value are allocatedaccording to any restrictions on such value pages and then used to storethe new value corresponding to the key. The value pages used to storethe new value are referred to as the new value page chain.

At 908, associating data between a key page and the new set of one ormore value pages is stored. In some embodiments, the physical address ofthe page in the KVPM that is the (first) page of the new value pagechain is stored in the OOB space (e.g., in the “pointer to current valuepage chain” field of the key page in FIG. 6A) of the key page. In someembodiments, the physical address of the page in the KVPM that is thenext page, if any, of the new value page chain is stored in the OOBspace (e.g., in the “pointer to next value page in chain” field of thevalue page in FIG. 6B) of a value page of the new value page chain. Insome embodiments, the physical address of the page in the KVPM that is aprevious page, if any, of the new value page chain is stored in the OOBspace (e.g., in the “back pointer” field of the value page in FIG. 6B)of a value page that is other than the first page of the new value pagechain. In some embodiments, the physical address of the page in the KVPMthat is the key page is stored in the OOB space (e.g., in the “backpointer” field of the value page in FIG. 6B) of the first value page ofthe new value page chain. In some embodiments, a version number of thenew value page chain that indicates that the value page chain is thecurrent version of the value corresponding to the key is stored in theOOB space (e.g., in the “version number of the value page chain” fieldof the value page in FIG. 6B) of each value page of the new value pagechain.

As will be described in further detail below, multiple out-of-placeupdates of values corresponding to a key will result in a linked list ofvalue page chains associated with different snapshot versions of valuescorresponding to the key.

At 910, associating data between the new set of one or more value pagesand a previous set of one or more value pages associated with storing aprevious value corresponding to the key is stored. In some embodiments,the physical address of the page in the KVPM that is the (first) valuepage of the previous value page chain that stores the immediatelyprevious version of the value corresponding to the key is stored in the00B space (e.g., in the “pointer to previous version of value pagechain” field of the value page in FIG. 6B) of the first page of the newvalue page chain.

At 912, a page usage data structure that tracks page allocation isupdated. The page usage data structure is updated to indicate that thepages associated with storing the new value are now used.

FIG. 10 is a flow diagram showing an example of a process for performinga put key operation. In some embodiments, process 1000 is implemented atKVPM 304 of FIG. 3. In some embodiments, at least part of process 400 ofFIG. 4A or steps 422-426 of process 420 of FIG. 4C are implemented usingprocess 1000. In some embodiments, process 700 of FIG. 7, process 800 ofFIG. 8, and process 900 of FIG. 9 may be implemented at least in partusing process 1000.

At 1002, a key and a corresponding value are received. A key and acorresponding value are received in a put key operation (e.g., from anapplication).

At 1004, a page is read for the key. For example, at least a portion ofthe key may be hashed to a physical address of the KVPM. That page isread to determine whether the key is stored in that page.

At 1006, it is determined whether the page includes the key. Thedetermined physical address of the KVPM is read to determine whether thekey is stored in that page. In the event that the key is not stored inthat page, control is transferred to 1024. Otherwise, in the event thatthe key is stored in that page, it is determined that an update to avalue of the previously stored key is to occur and the control istransferred to 1008.

At 1024, it is determined whether the page is free. In the event thatthe page that is checked for the key does not include the key and isalso not free (e.g., the page stores another key), then a hash collisionmay have occurred and a predetermined probe sequence is used todetermine the next page in the KVPM to check. At 1034, it is determinedwhether a maximum number of attempts of the probe sequence has beenreached. In some embodiments, a maximum on the number of attempts of theprobe sequence (e.g., five attempts) may be established to determinewhen to return a failure message in response to the put key operation.In the event that it is determined that the maximum number of attemptsof the probe sequence has not been reached, then a next probe location(e.g., a next page of the KVPM) is checked for the key at 1032 andcontrol is then returned to 1004. In the event that it is determinedthat the maximum number of attempts of the probe sequence has beenreached, process 1000 ends and a failure message is optionally returnedto the entity (e.g., application) from which the put key operation wasreceived.

In the event that the page that is checked for the key does not includethe key but is free, then control is transferred to 1026

At 1026, new page(s) are allocated for the value. As many pages of theKVPM that are needed to store the value are allocated.

At 1028, a new key page is written. The free page determined at 1024 iswritten with the key of the put key operation and becomes the new keypage.

At 1030, new value page(s) are written. The newly allocated page(s) of1026 are written with the value of the put key operation and become anew value page chain.

At 1008, it is determined whether an in-place update or an out-of-placeupdate is to be performed. Various mechanisms can be used to determinethe mode of update. For example, the mode of update may be determinedbased on the value set of a global variable. In various embodiments, themode of update may be set at various granularities (e.g., at theoperation level or at the application level). In the event that anin-place update is to be performed, control is transferred to 1010.Otherwise, in the event that an out-of-place update is to be performed,control is transferred to 1016.

At 1010, additional pages are allocated for a new value, if needed. Ifthe new value corresponding to the key will require mores pages to storethan the previous value, then one or more additional pages of the KVPMare to be allocated.

At 1012, at least some of previous value pages are overwritten. At leastsome of value pages that were used to store the previous version of thevalue corresponding to the key are overwritten with the new value.

At 1014, unused one(s) of the previous value pages are released. Iffewer than all of the value pages that were used to store the previousversion of the value corresponding to the key are not overwritten, theyare released (e.g., marked as unused so that they may be allocatedagain).

At 1016, new page(s) for storing the new value are allocated. As manynew pages that are needed to store the new value corresponding to thekey are allocated.

At 1018, the new value is written to the new page(s).

At 1020, the new page(s) are linked to previous page(s) associated withstoring a previous value corresponding to the key. A pointer pointing tothe (first page of) the new page(s) storing the new value is stored inat least one page of the previous page(s) storing the previous value.

At 1022, the new page(s) are linked to the key page. A pointer pointingto the (first page of) the new page(s) storing the new value is storedin the key page. A pointer pointing to the key page is stored in atleast one of the new page(s) storing the new value.

FIGS. 11, 12 and 13 describe examples of key-value pairs stored in pagesof a KVPM using put key operations.

FIG. 11 shows an example of the result of using the put key operation tostore a new key-value pair in pages of the KVPM. In the example of FIG.11, a put key operation to store the new key-value pair of“Key1-dog,cat” is received (e.g., from an application). The key “Key1”is hashed to physical address “012” of the KVPM. When the page in KVPMassociated with the physical address of “012” is checked for whether itis free, it is determined that the page is already used to store “Key8.”As such, a hash collision is determined and a probe sequence thatdictates, in this example, that the page that is two pages down (at“014”) relative to the current page at “012” in the KVPM should bechecked next. The page associated with physical address “014” is checkednext and is determined to be free and so “Key1” is stored in that page.It is determined that two pages are needed to store the value “dog,cat”and so two unused pages are allocated for storing “dog,cat.” In theexample of FIG. 11, the allocated pages at the physical address of “016”and “018” are respectively used to store the “dog” and “cat” portions ofthe value. A pointer that points to the page that stores “dog” (thefirst value page, the page at physical address “016”) is stored in thekey page (the page at physical address “014”). A pointer that points tothe page that stores “cat” (the second value page, the page at physicaladdress “018”) is stored in the page that stores “dog” (the first valuepage, the page at physical address “016”). The pages at physicaladdresses “016” and “018” that store “dog” and “cat” respectively arereferred to as a value page chain. The page usage data structure mayalso be updated to indicate that previously unused pages at physicaladdresses “014,” “016,” and “018” are now used.

FIG. 12 shows an example of the result of using the put key operation tostore a new value for a previously stored key in pages of the KVPM inthe in-place update mode. After the key-value pair of FIG. 11 wassuccessfully stored at the KVPM, another put key operation that includesthe key-value pair of “Key1-owl” is received (e.g., from anapplication). It is determined that an in-place update is to beperformed based on an appropriate mechanism. The key “Key1” is onceagain hashed to physical address “012” of the KVPM (using the samehashing function that was used in the example of FIG. 11). Again, whenthe page in KVPM associated with the physical address of “012” ischecked for whether it is free, it is determined that the page isalready used to store “Key8.” Similarly, a probe sequence that dictates,in this example, that the page that is two pages down (at “014”)relative to the current page at “012” in the KVPM should be checked nextis followed. The page associated with physical address “014” is checkednext and is determined that the page does include the expected key,“Key1.”

Given that an in-place update is to be performed, the pointer stored inthe key page at physical address “014” is read and used to identify thefirst key page at physical address “016.” Storing the new value of “owl”requires only one page and so only the first key page at physicaladdress “016” is overwritten with the new value of “owl.” As such, thesecond key page at physical address “018” is no longer used and will bemarked as unused in the page usage data structure. Further, the pointerto the value page at physical address “018” that was previously storedat the value page at physical address “016” (now storing “owl”) isremoved.

FIG. 13 shows an example of the result of using the put key operation tostore a new value for a previously stored key in pages of the KVPM inthe out-of-place update mode. After the key-value pair of FIG. 11 wassuccessfully stored at the KVPM, another put key operation that includesthe key-value pair of “Key1-owl” is received (e.g., from anapplication). It is determined that an out-of-place update is to beperformed based on an appropriate mechanism. The key “Key1” is onceagain hashed to physical address “012” of the KVPM (using the samehashing function that was used in the example of FIG. 11). Again, whenthe page in KVPM associated with the physical address of “012” ischecked for whether it is free, it is determined that the page isalready used to store “Key8.” Similarly, a probe sequence that dictates,in this example, that the page that is two pages down (at “014”)relative to the current page at “012” in the KVPM should be checked nextis followed. The page associated with physical address “014” is checkednext and is determined that the page does include the expected key,“Key1.”

Given that an out-of-place update is to be performed, it is determinedthat the new value of “owl” requires only one page and so one unusedpage of the KVPM is allocated to store “owl.” In the example of FIG. 13,the page at physical address “020,” which was previously unused, isallocated for storing “owl.” Because an out-of-place update hasoccurred, “owl” is the current version of the value corresponding to key“Key1” and “dog,cat” is now a previous snapshot version of the valuecorresponding to key “Key1” that is still maintained. A pointer thatpoints to the page that stores “owl” (the current version of the value,the page at physical address “020”) is stored in the key page (the pageat physical address “014” to replace the pointer that points to the pagethat stores “dog” (the first value page, the page at physical address“016”)). Furthermore, a pointer that points to the page that stores“dog” (the first value page of the previous snapshot version of the key,the page at physical address “016”) is stored in the current value page(the page at physical address “020”). In the example of FIG. 13, thereis only one snapshot version for key “Key1” so it is referred to as“Snapshot version 1.”

Because each snapshot version of a value corresponding to the key pointsto an earlier snapshot version of the value, if any, the snapshotversions of the value corresponding to the key form a chain (e.g., alinked list) in the KVPM. Therefore, releasing a snapshot version issimilar to deleting an element in a linked list. For example, a snapshotversion may be selected to be released by a user or an applicationbecause it is no longer needed. The chain (linked list) of snapshotversions corresponding to the value of a key are walked through, thesnapshot version that is immediately younger than (i.e., the snapshotversion that was generated immediately later than) the to-be-releasedsnapshot version is identified, and a pointer from that identifiedsnapshot version is updated to point to the snapshot version that isimmediately older than (i.e., the snapshot version that was generatedimmediately before) the to-be-released snapshot version. Afterwards, thepages occupied by the released snapshot version are released. FIG. 14,below, describes an example process of releasing an identified snapshotversion.

FIG. 14 is a flow diagram showing an embodiment of releasing a snapshotversion of a value corresponding to a stored key. In some embodiments,process 1400 is implemented at KVPM 304 of FIG. 3.

At 1402, a request to release an identified snapshot version for a keyis received. A request to release an identified snapshot version of avalue corresponding to a key is received. For example, the request maybe received from an application.

At 1406, it is determined whether the identified snapshot version is themost recent snapshot version. In various embodiments, the most recentsnapshot version is the version of the value that is immediately olderthan (i.e., the snapshot version that was generated immediately before)and pointed to by the value page chain that is storing the current valuecorresponding to the key. It is determined at 1406 whether the snapshotversion to be released is the version of the value that is immediatelyolder than and pointed to by the value page chain that is storing thecurrent value corresponding to the key.

In the event that the identified snapshot version is the most recentsnapshot version, control is transferred to 1416. At 1416, dataassociating a page associated with storing a current value correspondingto the key with a set of pages associated with storing a second mostrecent snapshot version for the key is stored. Given that the snapshotversion to be released is the version of the value that is immediatelyolder than and pointed to by the value page chain that is storing thecurrent value corresponding to the key, a pointer that points to thevalue page chain associated with storing the snapshot version that isimmediately older than the identified snapshot version is stored in thecurrent value page chain.

At 1408, a first snapshot version for the key that comprises animmediately younger snapshot version for the key relative to theidentified snapshot version is determined. The snapshot version that isimmediately younger than (i.e., the snapshot version that was generatedimmediately later than) and points to the identified snapshot version isdetermined.

At 1410, a second snapshot version for the key that comprises animmediately older snapshot version for the key relative to theidentified snapshot version is determined. The snapshot version that isimmediately older than and is pointed to by the identified snapshotversion is determined.

At 1412, data associating a page associated with storing the immediatelyyounger snapshot version with a page associated with storing theimmediately older snapshot version for the key is stored. The page inthe value page chain that stores the snapshot version that isimmediately younger than the identified snapshot version is updated topoint to the value page chain that stores the snapshot version that isimmediately older than the identified snapshot version.

At 1414, a page usage data structure is updated to free one or morepages that are associated with storing the identified snapshot version.

FIGS. 15 and 16 show examples of releasing a snapshot version of a valuecorresponding to a key.

FIG. 15 shows an example of pointers between a key page and value pagesassociated with storing various versions of the value corresponding to akey. As shown in FIG. 15, series of pages 1500 is a set of pages thatstores a key and multiple versions of values that correspond to thatkey. Key page 1502 in the KVPM is shown to point to the page(s) in theKVPM that are associated with storing current version 1504 of the valuecorresponding to the key stored in key page 1502. The page(s) in theKVPM that store current version 1504 of the value corresponding to thekey stored in key page 1502 then point to the page(s) in the KVPM thatare associated with storing snapshot version 4 1506, which is thesnapshot version that is immediately older than current version 1504, ofthe value corresponding to the key stored in key page 1502. The page(s)in the KVPM that store snapshot version 4 1506 of the valuecorresponding to the key stored in key page 1502 then point to thepage(s) in the KVPM that are associated with storing snapshot version 31508, which is the snapshot version that is immediately older thansnapshot version 4 1506, of the value corresponding to the key stored inkey page 1502. The page(s) in the KVPM that store snapshot version 31508 of the value corresponding to the key stored in key page 1502 thenpoint to the page(s) in the KVPM that are associated with storingsnapshot version 2 1510, which is the snapshot version that isimmediately older than snapshot version 3 1508, of the valuecorresponding to the key stored in key page 1502. The page(s) in theKVPM that store snapshot version 2 1510 of the value corresponding tothe key stored in key page 1502 then point to the page(s) in the KVPMthat are associated with storing snapshot version 1 1512, which is thesnapshot version that is immediately older than snapshot version 2 1510,of the value corresponding to the key stored in key page 1502.

In the examples of FIGS. 15 and 16, snapshot version 3 1508 is to bereleased. For example, a request to remove snapshot version 3 1508 isreceived from an application.

FIG. 16 shows an example of releasing a snapshot version of a valuecorresponding to a key. Applying a process such as process 1400 of FIG.14, snapshot version 3 1508 may be released by identifying the snapshotversion that is immediately older than snapshot version 3 1508,identifying the snapshot version that is immediately younger thansnapshot version 3 1508, and updating the pointer from snapshot version4 1506 to point to the page(s) of snapshot version 2 1510 (instead ofthe pages of snapshot version 3 1508). In addition, the page(s) in theKVPM that are associated with storing snapshot version 3 1508 of thevalue corresponding to the key are marked as unused at the page usagedata structure. Therefore, to remove snapshot version 3 1508, the linkedlist of pages in the KVPM that store various snapshot versions of avalue corresponding to a key is modified to no longer point to theremoved snapshot version 3 1508.

FIG. 17 is a flow diagram showing an embodiment of a process forretrieving from a KVPM a stored value corresponding to a key. In someembodiments, process 1700 is implemented at KVPM 304 of FIG. 3. In someembodiments, process 410 of FIG. 4B or steps 428 and 430 of process 420of FIG. 4C are implemented at least in part using process 1700.

At 1701, a get key operation including a key is received. For example,the get key operation is received from an application. In process 1700,the received get key operation is configured to retrieve from the KVPM astored value corresponding to a given key.

At 1702, a physical address of a KVPM is determined based at least inpart on the key. In various embodiments, a hash function (e.g., formula(1) above) is used to compute a physical address of the KVPM based on atleast a portion of the key.

At 1704, a page in the KVPM that stores the key is determined based atleast in part on the physical address. The page in the KVPM that isassociated with the physical address that is determined by the hashfunction is first checked for whether it contains the key included inthe put key operation. In some embodiments, the predetermined probesequence (the same one that was used for storing the key in the put keyoperation) is used to select the next page to check for the key includedin the get key operation, until the appropriate key page is located.

At 1706, associating data stored in the page associated with storing thekey is used to determine a set of one or more pages in the KVPMassociated with storing a value corresponding to the key. The pointerthat points to the (first) page of the value page chain associated withstoring the current version of the value is read from the 00B space ofthe key page and used to locate the value page chain associated withstoring the current version of the value.

At 1708, the value is obtained from the set of one or more pages. Theone or more pages of the value page chain that is used to store thevalue corresponding to the given key are read to retrieve the value.

At 1710, page metadata is updated. For example, statistics informationthat is stored in the key page describing the number of instances thatthe value corresponding to the key is read (or written) is updated.

FIG. 18 is a flow diagram showing an example of a process for performinga get key operation. In some embodiments, process 1800 is implemented atKVPM 304 of FIG. 3. In some embodiments, process 410 of FIG. 4B or steps428 and 430 of process 420 of FIG. 4C are implemented at least in partusing process 1800. In some embodiments, process 1700 of FIG. 17 may beimplemented at least in part using process 1800.

At 1802, a key is received. A key is received in a get key operation(e.g., from an application).

At 1804, a page is read for the key. In various embodiments, a hashfunction (e.g., formula (1) above) is used to compute a physical addressof the KVPM based on at least a portion of the key. The page of the KVPMassociated with the hashed-to physical address is read to check for thekey.

At 1806, it is determined whether the page includes the key. Thedetermined physical address of the KVPM is read to determine whether thekey is stored in that page. In the event that the key is not stored inthat page, control is transferred to 1810. Otherwise, in the event thatthe key is stored in that page, control is transferred to 1808.

At 1808, page(s) that store a corresponding value are read. The 00Bspace of the key page is read for the pointer to the (first page of the)value page chain associated with storing the current version of thevalue corresponding to the given key of the get key operation.

At 1810, it is determined whether the page is free. In the event thatthe page that is checked for the key does not include the key and isalso not free (e.g., the page stores another key), then a hash collisionmay have occurred and a predetermined probe sequence is used todetermine the next page in the KVPM to check. In various embodiments,the same predetermined probe sequence is used to locate the key page inthe get key operation that was used to store the key in the put keyoperation. At 1814, it is determined whether a maximum number ofattempts of the probe sequence has been reached. In some embodiments, amaximum on the number of attempts of the probe sequence (e.g., fiveattempts) may be established to determine when to return a failuremessage in response to the get key operation. In the event that it isdetermined that the maximum number of attempts of the probe sequence hasnot been reached, then a next probe location (e.g., a next page of theKVPM) is checked for the key at 1812 and control is then returned to1804. In the event that it is determined that the maximum number ofattempts of the probe sequence has been reached, process 1800 ends and afailure message is optionally returned to the entity (e.g., application)from which the get key operation was received. In the event that thepage that is checked for the key does not include the key but is free,process 1800 ends and a failure message is optionally returned to theentity (e.g., application) from which the get key operation wasreceived.

FIG. 19 shows an example of the result of using the get key operation toretrieve from the pages of a KVPM a stored value corresponding to agiven key. In the example of FIG. 19, a get key operation to retrievethe current version of the value corresponding to a given key, “Key1,”is received (e.g., from an application). The key “Key1” is hashed tophysical address “012” of the KVPM. When the page in KVPM associatedwith the physical address of “012” is checked for whether it is free, itis determined that the page is already used to store “Key8.” As such, ahash collision is determined and a probe sequence that dictates, in thisexample, that the page that is two pages down (at “014”) relative to thecurrent page at “012” in the KVPM should be checked next. The pageassociated with physical address “014” is checked next and is determinedto store “Key1” so the page associated with physical address “014” isidentified as the key page. The pointer to the page associated with thecurrent version of the value is read from the OOB space of the key page.This pointer points to the page associated with physical address “020”and so the value “owl” is read from the page associated with physicaladdress “020” and returned to the requesting application. While Snapshotversion 1 of the value corresponding to “Key1” is not requested by theget key operation, it is noted that the pages that store Snapshotversion 1 of the value may be received, when requested, via a pointerstored in the OOB space of the page associated with physical address“020.”

It should be mentioned that the get key operation can be used toretrieve snapshot versions as well. The API can be extended with anoptional version number argument to specify which snapshot version to beretrieved. If the version number is absent (not specified), the defaultservicing of the operation is to retrieve the current version.

As described above, in KVPM, applications access data with keysdirectly, without the need of calculating LBAs from keys and/or mappingan LBA to a PBA. Furthermore, KVPM's ability to perform in-place updateresults in simple address mapping; a key from an application can beeasily mapped to a memory page on a persistent storage media device andthere is no need to maintain complex, expensive indirection system likein Flash-based SSD (e.g., Flash Translation Layer andLogical-to-Physical table). In addition, there is no need for compactionor garbage collection, which reduces overhead and read/writeamplification. Unlike Flash-based SSD storage, key-value records arestored in KVPM directly so there are no multiple levels of files, and nowasted space caused by duplicated data. Various embodiments describedherein also provide native, fine-grained, and on-demand snapshotting.When updating an existing key-value record, a new value can be eitherupdated in-place if the previous version is no longer needed, or it canbe written to newly allocated pages with a link to the previous version.Whether to perform an update in-place or out-of-place may be determineddynamically at runtime and at a per-key basis. This not only reducesoverhead from software-based snapshotting, but also avoids wasted spacefrom unnecessary snapshot copies.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system, comprising: a persistent memory mediathat supports memory persistence and in-place updating; and a processorcoupled to the persistent memory media, configured to: perform a put keyoperation, including to: obtain a key and a corresponding value;directly store the key at the persistent memory media using a physicaladdress that is determined based at least in part on the key; anddirectly store the value at the persistent memory media.
 2. The systemof claim 1, wherein the processor is further configured to: generate apage usage data structure at boot time; and dynamically update the pageusage data structure in response to updates in page allocation in thepersistent memory media during runtime.
 3. The system of claim 1,wherein to directly store the key to the persistent memory mediacomprises to determine the physical address without requiring atranslation of a logical address associated with the key to the physicaladdress.
 4. The system of claim 1, wherein to directly store the key tothe persistent memory media comprises to determine the physical addressbased at least in part on hashing at least a portion of the key to thephysical address.
 5. The system of claim 1, wherein to directly storethe key to the persistent memory media using the physical address thatis determined based at least in part on the key comprises to: determinethat a page associated with the physical address in the persistentmemory media is unused; store the key at the page associated with thephysical address in the persistent memory media; and store associatingdata between the page associated with the physical address in thepersistent memory media associated with storing the key and a set ofpages in the persistent memory media associated with storing the value.6. The system of claim 1, wherein to directly store the key to thepersistent memory media using the physical address that is determinedbased at least in part on the key comprises to: determine that a pageassociated with the physical address in the persistent memory mediastores another key; select a new page in the persistent memory mediabased at least in part on a probe sequence; determine that the new pagein the persistent memory media is unused; store the key at the new pagein the persistent memory media; and store associating data between thenew page in the persistent memory media associated with storing the keyand a set of pages in the persistent memory media associated withstoring the value.
 7. The system of claim 1, wherein the put keyoperation comprises a first put key operation, wherein the valuecomprises a previous value, and wherein the processor is furtherconfigured to: perform a second put key operation, including to: obtainthe key and a corresponding new value; in response to a determinationthat a page associated with the physical address in the persistentmemory media stores the key, determine to perform an in-place update;and overwrite, with the new value, at least a portion of a set of pagesin the persistent memory media associated with storing the previousvalue.
 8. The system of claim 1, wherein the put key operation comprisesa first put key operation, wherein the value comprises a previous value,wherein the previous value is stored in a previous set of pages in thepersistent memory media, and wherein the processor is further configuredto: perform a second put key operation, including to: obtain the key anda corresponding new value; in response to a determination that a pageassociated with the physical address in the persistent memory mediastores the key, determine to perform an out-of-place update; write thenew value in a new set of pages in the persistent memory media; storeassociating data between the page associated with the physical addressin the persistent memory media and the new set of pages in thepersistent memory media; and store associating data between the new setof pages in the persistent memory media and the previous set of pages inthe persistent memory media.
 9. The system of claim 1, wherein theprocessor is further configured to: receive a request to release anidentified snapshot version for the key; determine an immediatelyyounger snapshot version for the key relative to the identified snapshotversion; determine an immediately older snapshot version for the keyrelative to the identified snapshot version; store data associating oneor more pages in the persistent memory media associated with storing theimmediately younger snapshot version with one or more pages in thepersistent memory media associated with storing the immediately oldersnapshot version; and update a page usage data structure to indicatethat one or more pages in the persistent memory media associated withstoring the identified snapshot version are unused.
 10. A method,comprising: receiving a put key operation including a key and acorresponding value; directly storing the key at a persistent memorymedia using a physical address that is determined based at least in parton the key, wherein the persistent memory media supports memorypersistence and in-place updating; and directly storing the value at thepersistent memory media.
 11. The method of claim 10, wherein directlystoring the key to the persistent memory media comprises determining thephysical address without requiring a translation of a logical addressassociated with the key to the physical address.
 12. The method of claim10, wherein directly storing the key to the persistent memory mediacomprises determining the physical address based at least in part onhashing at least a portion of the key to the physical address.
 13. Themethod of claim 10, wherein directly storing the key to the persistentmemory media using the physical address that is determined based atleast in part on the key comprises: determining that a page associatedwith the physical address in the persistent memory media is unused;storing the key at the page associated with the physical address in thepersistent memory media; and storing associating data between the pageassociated with the physical address in the persistent memory mediaassociated with storing the key and a set of pages in the persistentmemory media associated with storing the value.
 14. The method of claim10, wherein directly storing the key to the persistent memory mediausing the physical address that is determined based at least in part onthe key comprises: determining that a page associated with the physicaladdress in the persistent memory media stores another key; selecting anew page in the persistent memory media based at least in part on aprobe sequence; determining that the new page in the persistent memorymedia is unused; storing the key at the new page in the persistentmemory media; and storing associating data between the new page in thepersistent memory media associated with storing the key and a set ofpages in the persistent memory media associated with storing the value.15. The method of claim 10, wherein the put key operation comprises afirst put key operation, wherein the value comprises a previous value,and further comprising: performing a second put key operation,including: obtaining the key and a corresponding new value; in responseto a determination that a page associated with the physical address inthe persistent memory media stores the key, determining to perform anin-place update; and overwriting, with the new value, at least a portionof a set of pages in the persistent memory media associated with storingthe previous value.
 16. The method of claim 10, wherein the put keyoperation comprises a first put key operation, wherein the valuecomprises a previous value, wherein the previous value is stored in aprevious set of pages in the persistent memory media, and furthercomprising: performing a second put key operation, including: obtainingthe key and a corresponding new value; in response to a determinationthat a page associated with the physical address in the persistentmemory media stores the key, determining to perform an out-of-placeupdate; writing the new value in a new set of pages in the persistentmemory media; storing associating data between the page associated withthe physical address in the persistent memory media and the new set ofpages in the persistent memory media; and storing associating databetween the new set of pages in the persistent memory media and theprevious set of pages in the persistent memory media.
 17. The method ofclaim 10, further comprising: receiving a request to release anidentified snapshot version for the key; determining an immediatelyyounger snapshot version for the key relative to the identified snapshotversion; determining an immediately older snapshot version for the keyrelative to the identified snapshot version; storing data associatingone or more pages in the persistent memory media associated with storingthe immediately younger snapshot version with one or more pages in thepersistent memory media associated with storing the immediately oldersnapshot version; and updating a page usage data structure to indicatethat one or more pages in the persistent memory media associated withstoring the identified snapshot version are unused.
 18. A computerprogram product, the computer program product being embodied in anon-transitory computer readable storage medium and comprising computerinstructions for: receiving a put key operation including a key and acorresponding value; directly storing the key at a persistent memorymedia using a physical address that is determined based at least in parton the key, wherein the persistent memory media supports memorypersistence and in-place updating; and directly storing the value at thepersistent memory media.
 19. A system, comprising: a persistent memorymedia that supports memory persistence and in-place updating; and aprocessor coupled to the persistent memory media, configured to performa get key operation, including to: obtain a key; and directly access thepersistent memory media using a physical address that is determinedbased at least in part on the key to retrieve a stored valuecorresponding to the key.
 20. The system of claim 19, wherein todirectly access the persistent memory media comprises to determine thephysical address without requiring a translation of a logical addressassociated with the key to the physical address.
 21. The system of claim19, wherein to directly store the key to the persistent memory mediacomprises to determine the physical address based at least in part onhashing at least a portion of the key to the physical address.
 22. Thesystem of claim 19, wherein to directly access the persistent memorymedia using the physical address that is determined based at least inpart on the key to retrieve the stored value that corresponds to the keycomprises to: determine that a page associated with the physical addressin the persistent memory media stores the key; and use associating datastored in the page associated with the physical address in thepersistent memory media to determine a set of pages in the persistentmemory media at which the value is stored.
 23. A method, comprising:receiving a get key operation including a key; and directly accessing apersistent memory media using a physical address that is determinedbased at least in part on the key to retrieve a stored valuecorresponding to the key, wherein the persistent memory media supportsmemory persistence and in-place updating.
 24. The method of claim 23,wherein directly accessing the persistent memory media using thephysical address that is determined based at least in part on the key toretrieve the stored value that corresponds to the key comprises:determining that a page associated with the physical address in thepersistent memory media stores the key; and using associating datastored in the page associated with the physical address in thepersistent memory media to determine a set of pages in the persistentmemory media at which the stored value is stored.
 25. A computer programproduct, the computer program product being embodied in a non-transitorycomputer readable storage medium and comprising computer instructionsfor: receiving a get key operation including a key; and directlyaccessing a persistent memory media using a physical address that isdetermined based at least in part on the key to retrieve a stored valuecorresponding to the key, wherein the persistent memory media supportsmemory persistence and in-place updating.